1. Field of the Invention
This invention relates to optical diffraction gratings, and more specifically to a method for forming refractive index gratings in photosensitive optical fibers, and the resulting fibers.
2. Description of the Related Art
Optical fiber diffraction gratings are useful for optical communications devices such as single-mode fiber lasers, hybrid semiconductor-fiber lasers, mode converters, optical filters and fiber sensors.
Fiber gratings typically consist of a refractive index grating written in the core of a photosensitive fiber, as described in G. Meltz et al., "Formation of Bragg gratings in optical fibers by a transverse holographic method", Optics Letters, vol. 14, no. 15, August 1989, pages 823-825. The photosensitive fiber is typically a silicon fiber with a core that is doped with a material that makes its index of refraction sensitive to its history of exposure to optical radiation of a given wavelength. For example, a silicon fiber doped with germanium exhibits an intense 35 nm wide absorption band centered at 244 nm.
In the transverse process described by Meltz, the diffraction grating is written in the core of the fiber by exposing it to a two-beam interference pattern. The wavelength of the two beams are chosen to coincide with the absorption band in the fiber (for example, 244 nm) and the beams illuminate the core from the side of the fiber. The two interfering beams create sinusoidal light and dark interference fringes in the fiber, which cause a corresponding sinusoidal variation in the refractive index of the fiber core (an index grating). The sinusoidal index grating has the same period as the optical interference fringes. The period of the interference fringes, and hence the period of the resulting index grating, is dependent on the writing angle between the two optically interfering beams and their wavelength. Since there is typically a constraint on the wavelength of the writing beams (it must coincide with the absorption band in the fiber), the index grating period is typically controlled by varying the angle between the two writing beams.
Another method of forming index gratings in fibers is described in Dana Z. Anderson et al., "Phase-Mask Method for Volume Manufacturing of Fiber Phase Gratings", Proceedings of the Optical Fiber Conference, February 1993, paper PD16-1, pages 68-70. In this method a single source beam is passed through a phase mask (a phase grating), which diffracts the beam into multiple diffraction orders. The fiber is positioned in close proximity to (but not in direct contact with) the phase mask. The diffracted orders, which have the same function as the writing beams in the Meltz method, interfere in the fiber core and produce a sinusoidal index grating with a period that is equal to the phase mask grating period. With this method, the index grating period will always be equal to the phase mask grating period, regardless of the angle that the source beam makes with the phase mask.
Regardless of which exposure method is used, the fiber index grating will reflect light at its Bragg wavelength, which can be expressed with the equation .lambda..sub.B = 2n.LAMBDA./N, where .lambda..sub.B is the Bragg wavelength (the wavelength reflected by the grating), n is the index of refraction of the fiber, .LAMBDA. is the index grating period and N is the grating order. The sinusoidal gratings produced by the above described method are low order gratings with a grating period that is no larger than twice the primary design wavelength .lambda..sub.D (the primary wavelength that the grating is designed to reflect). These low order gratings reflect light at the fundamental order or, at most, the second order. As the grating period is increased relative to the primary design wavelength (resulting in a higher order grating), less light is reflected at both the fundamental order and at the higher orders for a given grating length.
One can compensate for this phenomenon by increasing the total length of the fiber grating as the grating period is increased. However, some applications impose limits on the fiber length that can be used. In addition, for the interferometric method described by Meltz, costly large aperture precision optical elements would have to be used to form long high order fiber gratings. In the phase mask method described by Anderson, the length of the grating is limited by the length over which the diffracted orders overlap in the fiber and the size of the mask. Chirp-free masks more than a few inches long are very difficult to obtain.
As a result of these limitations, prior fiber grating forming methods have only been used to form gratings that are low order with respect to the design wavelength (the grating period is no larger than twice the primary design wavelength). These gratings exhibit efficient reflectivity at no more than two grating orders (efficient reflectivity at two grating orders is very rare).
It would be advantageous to have high order fiber gratings with relatively high reflectivity at multiple orders. The ability to reflect light at multiple wavelengths using a single fiber grating would expand the flexibility and usefulness of fiber grating devices.